1. Field of the Invention
The present invention relates to EPROM and flash memory cells and, more particularly, EPROM and flash memory cells with source-side injection.
2. Description of the Related Art
An electrically-programmable read-only-memory (EPROM) cell and a flash memory cell are non-volatile memories that retain data stored in the cell after power to the cell has been removed. EPROM and flash memory cells principally differ from each other in that EPROM cells are erased with ultraviolet (UV) light, while flash cells are electrically erased.
FIG. 1 shows a cross-sectional view that illustrates a prior art EPROM or flash memory cell 100. As shown in FIG. 1, cell 100 includes spaced-apart n+ source and drain regions 112 and 114 which are formed in a p-type substrate 110, and a channel region 116 which is defined in substrate 110 between source and drain regions 112 and 114.
In addition, cell 100 also includes a layer of gate oxide 120 which is formed over channel region 116, and a floating gate 122 which is formed over gate oxide layer 120. Further, cell 100 additionally includes a layer of interpoly dielectric 124 which is formed over floating gate 122, and a control gate 126 which is formed over dielectric layer 124.
Cell 100 is programmed by applying a programming voltage to control gate 126, a drain voltage to drain region 114, and ground to source region 112. The programming voltage applied to control gate 126 induces a positive potential on floating gate 122 which, in turn, attracts electrons to the surface of channel region 116 to form a channel 130.
In addition, the source-to-drain voltage sets up an electric field which causes electrons to flow from source region 112 to drain region 114 via channel 130. As the electrons flow to drain region 114, the electric field, which has a maximum near drain region 114, accelerates these electrons into having ionizing collisions that form channel hot electrons near drain region 114.
A small percentage of the channel hot electrons are then injected onto floating gate 122 via gate oxide layer 120. Cell 100 is programmed when the number of electrons injected onto floating gate 122 is sufficient to prevent channel 130 from being formed when a read voltage is subsequently applied to control gate 124.
Since only a small percentage of the channel hot electrons are injected onto floating gate 122, channel hot electron programming provides a relatively low injection efficiency. One technique for increasing the injection efficiency is to create a source-to-drain electric field which, in addition to having a peak near the drain region, also has a peak near the source region. EPROM and flash memory cells which utilize an electric field which has a peak near the source region are typically referred to as having source side injection.
One way of forming an electric field which has a peak near both the source and drain regions, as described in U.S. Pat. No. 4,652,897 to Okuyama et al., is to use a low-density source region which is adjacent to the source region.
FIG. 2 shows a cross-sectional view that illustrates a prior-art, source-side injection EPROM or flash memory cell 200. FIG. 2 is similar to FIG. 1 and, as a result, utilizes the same reference numerals to designate the structures which are common to both cells.
As shown in FIG. 2, source-side cell 200 differs from cell 100 in that cell 200 also includes a n-type low-density source (LDS) region 210 which is formed between source region 112 and channel region 130. LDS region 210 has a dopant concentration which is less than the dopant concentration of source region 112.
In operation, cell 200 is programmed the same as cell 100 except that the presence of LDS region 210 causes the source-to-drain electric field to have peaks near both the LDS and drain regions 210 and 114. As a result, channel hot electrons are formed and injected onto floating gate 122 near both of these regions 210 and 114, thereby increasing the injection efficiency.
FIG. 3 shows a graph that illustrates the intensity of the electric field along the surface of the channel region between the drain and source regions of cell 200. As shown in FIG. 3, line LO illustrates that cell 200 has a peak in the intensity of the electric field near both the drain and source regions.
One disadvantage of cell 200, however, is that as a result of LDS region 210, cell 200 consumes significantly more silicon real estate than does cell 100. Thus, there is a need for an EPROM or flash memory cell that provides an increased injection efficiency with reduced cell size.